Field of the Invention
The invention pertains to the field of vehicle differential assemblies. More particularly, the invention pertains to a limited slip torque sensitive differential assembly.
Description of Related Art
A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, typically used in one of two ways. In one way, the differential receives one input and provides two outputs and in the other way, the differential combines two inputs to create an output that is the sum, difference, or average, of the inputs.
In automobiles and other wheeled vehicles, the term often applies to a “traditional” rear-wheel-drive car or truck with an “open” or limited slip differential, where the differential allows each of the driving wheels to rotate at different speeds.
Thus, for example, if a car is making a turn to the right, the left wheel will make more rotations because it has further to travel, and the right wheel will make fewer rotations, as it has less distance to travel. For example, the left wheel makes 12 rotations, and the right wheel makes 8 rotations.
When the vehicle is traveling in a straight line, there is no differential movement of the planetary system of gears other than the minute movements necessary to compensate for certain factors, which may include, but are not limited to, slight differences in wheel diameter and undulations in the road that make for a longer or shorter wheel path.
Differentials are also used to distribute torque between wheels in front axle drive applications.
Differentials may also be used to distribute torque among multiple axles, such as center box applications.
An undesirable side effect of a conventional differential is that it can limit traction under less than ideal conditions. The amount of traction required to propel the vehicle at any given moment depends on one or more factors, which may include, but is not limited to, the load at that instant, how heavy the vehicle is, how much drag and friction there is, the gradient of the road, and the vehicle's momentum.
The torque applied to each driving wheel is a result of the engine, transmission, and drive axles applying a torque against the resistance of the traction at that wheel. In lower gears and thus at lower speeds, and unless the load is exceptionally high, the drive train can supply as much torque as necessary, so the limiting factor becomes the traction under each wheel. It is therefore convenient to define traction as the amount of torque that can be generated between the tire and the road surface before the wheel starts to slip. If the torque applied to drive wheels does not exceed the threshold of traction, the vehicle is propelled in the desired direction and if not, then one or more wheels simply slip relative to the road surface.
With an open differential, each tire is allowed to rotate at a different speed from the other as soon as a tire atop a slippery surface exceeds the threshold of traction available to it. Additionally, once the traction threshold is broken and the tire experiences slip, the traction available also decreases in accordance with the laws of kinetic friction. Since an open differential limits total torque applied to both drive wheels to the amount utilized by the lower traction wheel multiplied by a certain factor, when one wheel is on a slippery surface, the total torque applied to the driving wheels becomes lower than the minimum torque required for vehicle propulsion. Thus, the vehicle's propulsion becomes limited.
In an open differential, since the input torque is split into equal halves to the two outputs, the maximum torque available of the vehicle is limited by the wheel with the less traction torque; i.e., twice the traction torque of the wheel with the less traction torque.
One improvement to an open differential is the electronic traction control system. It usually uses an anti-lock braking system (ABS) wheel speed sensor to detect a spinning wheel and applies the brake to that wheel. This progressively raises the reaction torque at that wheel, and the differential compensates by transmitting more torque through the other wheel. However, this process happens in reaction to a slipping wheel and therefore it is considered reactive.
Another alternative improvement to an open differential is the limited slip differential. In a limited-slip type differential, the wheel with more traction torque is capable of transmitting more torque output than the other wheel with less traction torque; the maximum available torque difference between the two wheels is called the torque bias. This torque bias has to be overcome before one wheel slips. Thus, compared with the electronic traction control system, it is considered proactive.
The limited-slip type of differential generally has several types. One type has a relatively fixed torque bias between the two outputs. This type generally includes the components of an open differential as well as biasing elements. Typically a spring is used in this type between the two side gears, between the side gears and differential housing, or between the two spider gears to generate relatively constant friction, such as shown in U.S. Pat. No. 7,270,026. To magnify the available friction or frictional torque, a multi-disk clutch pack is generally used, which allows extra torque to be sent to a wheel with higher resistance than available at the other driven wheel, when the limit of friction is reached at that other wheel, such as shown in U.S. Pat. No. 3,331,262. One drawback of this type of differential is the constant wear at the frictional surfaces whenever there is relative sliding speed between them. This happens even when the vehicle makes a turn without needing significant power. Since this minimum frictional force is continually active to resist differentiation, the friction clutches tend to wear, resulting in a deterioration of intended differential performance. Another drawback is that since a minimum friction resistance must be overcome to allow any relative rotation between drive axles, there is a waste of energy. Still another drawback is that the available bias torque before one wheel slips is generally not very large, making the differential ineffective in torque demanding applications.
Another type of limited-slip differential has a relatively fixed ratio between the two torque outputs to the two wheels when they have different rotational speeds, which is called torque bias ratio. In this type, the available torque bias before one wheel slips is proportional to the input power or torque. Typically, a planet gearing system (either parallel axial such as shown in U.S. Pat. No. 3,706,239 or cross-axis helical such as shown in U.S. Pat. No. 2,859,641) is employed to link the two side gears so that the corresponding separation force and thrust force are generated against the housing. These separation and thrust forces cause the bias frictional torque to be generated. This frictional torque is generally proportional to the input torque or power, and thus the differential has a fixed torque bias ratio. The differential has no significant wear when the input power is close to zero, even when the two wheels have different rotational speeds, such as when the vehicle makes a turn without significant power. This type is also called a torque-sensitive limited slip differential. With more parts involved, the limited-slip differentials with fixed torque bias ratio are generally more complex and more difficult to manufacture, and therefore more expensive, than open differentials and multi-disk clutch type limited slip differentials. In addition, methods to retain axle shafts or axle retainers such as “C” clips are generally not compatible with the current torque-sensitive limited slip differentials. Generally, in order to upgrade to a torque-sensitive limited slip differential from an open differential, there is added cost to implement the axle retainer compatible features such as “C” clips, or the axle shafts have to be retained by other means.
Sometimes, in a more complicated differential, the features of the above two types (fixed torque bias and fixed torque bias ratio) are both used to obtain a more customized torque bias ratio or torque bias as a function of input power or input torque, as shown in U.S. Pat. No. 5,098,356, U.S. Pat. No. 5,362,284, and U.S. Pat. No. 8,287,418. The drawback is, of course, added complexity and cost.
There is still another type of differential called a speed sensitive limited slip differential. In this type, the torque bias is dependent upon the relative speed difference between the two wheels. When the speed difference is small, the torque bias is very small, almost similar to an open differential. When the speed difference is large, the torque bias is large. One example is the viscous type differential, as shown in U.S. Pat. No. 5,161,658. Viscous type differentials are typically less efficient than open differentials. For example, a viscous type differential loses or wastes some power during vehicle turning. Another drawback is that the viscous mechanism tends to deteriorate and the viscous differential behaves as a semi-open differential prior to the end of the vehicle's life expectancy. Other advanced speed sensitive limited slip differentials include Gerotor pump (as shown in U.S. Pat. No. 6,544,137) and electronic limited slip (such as in U.S. Pat. No. 5,215,506) differentials, in which the speed difference between the two wheels is sensed and a clamping force is actively generated in response, usually by hydraulics, to increase the torque bias between the two wheels. The drawback of these speed sensitive limited slip differentials is added complexity and cost.
Therefore, there is a need to reduce the cost and complexity of a limited slip differential.